By using a resonant coil transformer or high frequency transformer or similar device as it is sometimes called (also commonly referred to as a Tesla Coil), a high voltage field is produced in the coil's output antenna, a toroid or ball, which acts as a large capacitor. On a well tuned coil, sparks and bolts of artificial lightning will emit from the antenna. The length of these are dependent on the size of the coil and amount of power being used. In this form, they are often used for display and special effects in films.
The current sent to the coil is fed into large capacitors that store the energy to a much higher value, and release it to a primary coil, then induces a secondary coil to resonate and produce high frequency, high voltage at the secondary terminal antenna that is normally a ball or toroid. As mentioned above, the secondary terminal antenna also acts as a capacitor surrounded by an electromagnetic field (EMF) and will emit sparks and streaks of artificial lightning, the length of these being dependent on the size and construction of the coil.
If a larger antenna is used, the coil can be tuned to a frequency where no sparks or lightning emit, but still form a high voltage electromagnetic field (EMF) around the antenna. The depth and size of this field is also dependent on the size of the coil and power used.
The purpose for which Nikola Tesla invented this system was for wireless transmission of power over a large area.
This system is small enough that the EMF is insignificant, but can be adequately shielded within a Faraday cage or similar enclosure. This practice is well documented in many of Nikola Tesla patents, and these coils are readily available commercially. Tesla coils come in many configurations, and can be powered by AC or DC current.
The secondary winding of a Tesla coil is normally connected to ground, but by connecting another large antenna to the secondary coil as a ground plane it can also produce a high frequency magnetic field. The ground plane is larger than the top antenna but is shown in the drawings herein at a smaller scale to reflect a size similar to the top antenna, for ease of illustration only. The primary and secondary coils, as well as the antenna are sized to produce particular required frequencies and incorporate a safety circuit to conduct any excess current to ground.
A gas filled bulb, like an ordinary Fluorescent tube will light up if placed any where near a running coil, and will be brightest at the edge of the high voltage field. The other end of the tube can be to ground. The electrical current from the high voltage field passes through the tube to ground and causes the excitement of the gases which light the bulb. A small high voltage capacitor connected to the light bulb also enhances the light emitted from the bulb.
A standard fluorescent bulb has filaments at each end that burn out due to heat and fatigue. This does not occur with high frequency electricity as no filaments are needed, only a terminal in and out of the bulb; it does not generate heat and the bulb will last until it is broken or the gas escapes. A burnt out fluorescent bulb will work when subject to the field. Further, a much brighter light is possible by using special purpose made bulbs, by adjusting the composition of the gases and luminescent coating inside the bulb.
The area around the coil antenna limits the number of bulbs that can be illuminated, but high frequency high voltage power can be conducted along a very thin wire, with little or no power loss. By using very thin insulated wire as secondary antennae of various lengths, one end connected to each end of the fluorescent tube and the other ends suspended at the edge of the high frequency fields surrounding the coils antenna, the bulb will light up several feet away. This method allows many bulbs to be illuminated at once in the very small area taken by the ends of the secondary antennae. The number of bulbs that can be illuminated is only governed by how many secondary antennae that can be placed around the coil. For ease of illustration, the secondary antenna to the bulbs are shown in the drawings as separate wires, but can be incorporated into a circuit board (not shown) that plugs onto the bulb holder and terminates as a small pin with a very small footprint at the edge of the EMF.
The coil input and output remains constant and is not affected by the number of bulbs used. A good analogy of this effect is radio. A radio station's power covers an area governed by its output, and any number of radios switched on in that coverage area does not affect the station.
If several bulbs are placed together in line a small distance apart, a large area of light is created.